Last week the Saudi foreign minister, Prince Saud al-Faisal, met the US president, George Bush, in Washington for emergency talks. It is believed a congressional investigation into the September 11 2001 attacks implicates the Saudi government in financing the hijackers.

Bush has refused to publish the relevant 28 pages of the report, while Riyadh has angrily dismissed allegations that have not yet been officially levelled.

The prince's anger over Washington's secrecy is perhaps understandable. The US and Britain have a history of close cooperation with the Saudis, especially in military matters. But it is surprising, too, because Saudi Arabia is itself one of the world's most secretive states.

The criminal justice system there is "strictly secretive", according to Amnesty International, which has never received a Saudi response to concerns about human rights there. There is a fiercely enforced ban on political parties and no government data is published for public consumption, while human rights abuses and torture remain widespread.

Britain has kept an investigation about arms sales to Saudi Arabia secret for more than 10 years, without so much as a whimper of protest from the Saudis. Following allegations of corruption in massive arms deals between Saudi Arabia and Britain in the 1980s, the national audit office (NAO) launched an investigation in 1989. The resulting report remains the only NAO document never to have been published, despite promises to release it by New Labour while in opposition.

That may be about to change.

In his book Investigative Reporting: A Guide to Techniques, David Spark writes that uncovering the truth about alleged corruption and the Al Yamamah arms deal is the holy grail of investigative journalists. Now, a new investigation, this time into the NAO report and its secrecy, has been launched after a complaint by the Guardian to the parliamentary ombudsman, Ann Abraham.

The move could finally lead to the truth about a saga which involved one of the world's biggest arms companies, Margaret Thatcher and allegedly her son too, as well as playing a role in the downfall of Jonathan Aitken.

That's a lot in just one arms deal, but then it was a very big one indeed. Al Yamamah - which is Arabic for dove - was actually two separate deals over a decade. It has been described as "the biggest [UK] sale of anything to anyone".

Al Yamamah I, which was formally signed on February 17 1986, was worth an estimated £5bn at the time, and included the sale of Tornado and Hawk aircraft. British Aerospace - now BAe Systems - was the main contractor.

Newspapers called the deal "Maggie's bonanza" for the British jobs that would be saved, and because she personally lobbied the Saudi royals to seal the deal.

Al Yamamah II was signed on 5 July 1988. The shopping list included up to 50 more BAe Tornados, up to 60 more Hawks, more than 80 military helicopters and air bases. It was valued at not less than £10bn.

But just weeks after the first deal was signed, allegations emerged of sweetener payments totalling £600m to the Saudi royal family and to fixers. That the Saudis had far too few military pilots to fly so many aircraft should itself have raised suspicion.

The NAO was asked to investigate the corruption claims in 1989, but three years later the parliamentary public accounts committee (PAC) said it would not publish the findings. The chairman, the Labour MP Robert Sheldon, even refused to show the final report - which looked only at whether British officials received, or in particular, gave, payments - to committee members.

"We found no evidence of fraud or wrongdoing in the MoD or government departments," Mr Sheldon said.

In October 1994, MP Tam Dalyell submitted documents to parliament that he claimed proved Mark Thatcher was also involved. The same month, the Financial Times revealed the PAC had decided not to investigate allegations that Thatcher Jr had received up to £12m from the deal. There was no mention of Mark Thatcher in the report - it was beyond the committee's remit - and Sir Mark has always denied receiving this payment or exploiting his mother's connections in his business dealings.

If the Britain's relationship with the US is special, then our relationship with Saudi Arabia is staggering. The kingdom is a non-democratic state where last year "gross human rights violations continued", according to Amnesty International in their latest human rights report. Torture and ill-treatment remain rife, and executions are still meted out for "crimes" such as being gay, or for protesting against the closure of a mosque.

In 1997 British Aerospace was also the major contributor to a fund of £730,00, which averted the death sentence for two British nurses accused of drug smuggling in the kingdom. The payment eased a dangerous diplomatic row with the company's best customer.

According to the Labour government's own figures, in 2001 Britain authorised £20.5m worth of arms exports to Saudi Arabia. Last year, the figure rose to £29m. Oil sales, arms sales and the kingdom's strategic position in the Middle East clearly take precedence over our government's policies towards non-democratic, corrupt and human rights abusing states.

The ombudsman can't demand that the NAO report is published, though a spokeswoman said the government usually does what it is asked to do. The ombudsman's investigation will take months, so if the report confirms that there was widespread corruption in arms buying by Saudi royals, most of whom still run the country, evidence will not emerge in time to prevent Saudi arms buyers visiting London in September to attend a huge weapons fair.

Along with other questionable regimes, such as Indonesia, Colombia and Syria, the Saudi's were invited to shop for arms at the Defence Systems Equipment International (DSEI) exhibition in both 1999 and 2001.

The Ministry of Defence refused last week to say if they were invited this year, but if the last few years are anything to go by, the Saudi's will shop for arms at London Docklands over the week of 11 September. The rest of the world will be remembering the terrorist attacks of two years before, attacks it appears the US secret document will reveal could have been partly financed by the Saudi government.